At the end of Act 4 in Macbeth, why does Malcolm misrepresent his own character to Macduff?

The play is about betrayal and trust.  We see that Duncan
made a fatal mistake in trusting Macbeth.  Malcolm does not want to make the same
mistake in trusting Macduff, who wants him to return to Scotland.  Fearing a trap
(Remember Braveheart?), Malcolm must test Macduff's loyalty.  After
all, Macduff's presence in England is somewhat suspicious.  Malcolm wonders how Macduff
could leave his wife and children to come to England.  Perhaps Macbeth sent him and has
guaranteed protection for them. If Malcolm returns to Scotland with Macduff, how can he
be sure that some sort of ambush is not awaiting him.  He knows that Macbeth would like
to see him dead, and Macduff could be working for Macbeth.  So, he lies to Macduff and
tells him that he is the worst kind of man:  greedy, lustful, devoid of kingly virtues.
 If Macduff still wants him to return to Scotland, then Malcolm knows that it is a trap.
 But when Macduff hangs his head and walks away in frustration, Malcolm knows that
Macduff is sincere in his desire to have the rightful monarch on the throne of
Scotland.